The present invention relates to a method for bonding a member having metal with another member having metal or another metal material by using a filler material, without using a flux.
More specifically, this invention relates to a method for preventing the metal from being oxidized during the preservation of the member having metal, and making the bonding of the member having metal to the other member by using a filler material possible, without using a flux.
In more detail, this invention relates to a method for bonding electronic parts, which is appropriate for the bonding of electronic parts by using a filler material, and which is to be applied when mounting an electronic part on a substrate.
When a member including metal, such as various kinds of electronic parts, is exposed to air during preservation or transportation, the metal portion is easily oxidized, and the metal surface will be covered with an oxide layer. Once the oxide layer is formed in this way, it becomes difficult to bond the metal portion of this member to the other metal material or other member having metal by soldering or brazing.
Accordingly, a postflux has been used to remove the oxide layer on the metal surface. This postflux is a compound containing a halogen activator, and is an agent which is capable of removing the oxide layer on the metal surface when it is applied to the member to be bonded by a filler material.
However, there are some defects in the application of a postflux to electronic parts, such as the corrosion of the metal portion due to halogen compounds which remain on the metal portion of the electronic parts, and the deterioration of electrical insulation of the electronic parts.
Therefore, it has been necessary to clean the electronic parts with a solution containing an organic halogen based solvent, such as freon, or a surface active agent.
However, the technology of cleaning electronic parts also has the following defects:
First, halogen based solvents such as freon compounds including trichlorofluoromethane, trihaloethane and trihalomethan have a problem that they may destroy an ozone layer of the earth. The manufacture and use of these halogen based solvents is limited or is going to be limited by international treaties.
Secondly, when cleaning electronic parts with a solution containing a surface active agent, the waste water containing heavy metals and organic substances must be treated. It is also necessary to dry the electronic parts, thereby increasing costs.
Based on the above-described background, a flux containing a small amount of halogen has been suggested. For example, in the Japanese Patent Laid-Open (Kokai) Publication No. 1991-161191, a flux containing organic solvents, polyhydric alcohol, organic acids, and organic halogen activators, with the intention of decreasing the content of the halogen activators, is described. By using this flux, it is possible to restrict the amount of the remaining halogen, after bonding with a filler material, to 14 .mu.g/inch.sup.2 or less, and thus to omit the cleaning of the electronic parts after bonding with a filler material.
Further, there is a description in the MIL standard about a flux called R type which does not contain halogen activators and which consists essentially of an alcohol solution of rosin.
However, this type of flux has the following problems:
First of all, by using the R type flux, the flux may penetrate into the electronic parts and may hinder the electrical characteristics of the electronic parts.
There are other problems concerning both the flux containing a small amount of halogen and the R type flux as follows: The power of these fluxes to remove an oxide layer over the metal surface is small, and therefore, complete bonding cannot be attained. Also, the bonded strength turns out to be different for different electronic parts.
Under these circumstances, it has recently been attempted to place a metal member to be bonded in a container together with a drying agent, or to replace the air in the container with inert gas. However, it is not easy to completely replace the air in the container with inert gas such as nitrogen and argon gas. Even if the air replacement is completely done, oxygen and moisture penetrate through the packing material, which composes the container, into the container, and the oxygen content and the humidity within the container increase, causing the metal surface to be oxidized. It is also difficult to remove oxygen within the container by using a drying agent.
Accordingly, a flux has to be used in order to remove an oxide layer which is formed over the surface of the metal preserved in the container. However, since a usable flux is one containing a small amount of halogen activators or no halogen activators, the problem still remains that an oxide layer over the metal surface cannot be sufficiently removed at the end, and the bonding of the member including metal by a filler material is insufficient.